SHORT-TERM PROCESSES OF RADIONUCLIDE IMMOBILIZATION IN CEMENT - A CHEMICAL APPROACH

The ions released in solution by the constituents of cement (principally silicate, aluminate, OH and Ca ions) can combine with the anions and cations from nuclear wastes present in the mixing water to give very insoluble compounds that can fix these ions in the concrete matrix. In order to understand some of the particular physico-chemical processes involved in cement hydration in the presence of analogue elements, tricalcium silicate (C3S) was used instead of cement, which is too complex a mixture. It was found that the salt of a chemical analogue, a lanthanide salt, showed some accelerating effects when present in dilute amounts but resulted in large accelerating effects on C3S in stirred suspensions and pastes when present in concentrated amounts, In this case the corresponding cation was totally fixed in a very short time. Analysis of the liquid phase suggested that the fixing occurred partly in the form of silicate hydrate and partly as hydroxide. Microcalorimetric measurements confirmed the course of the chemical processes leading to the early fixation of La as a very insoluble compound. The capability of fixation of lanthanide, actinide and Co cations as hydroxides has also been estimated from the equilibrium concentrations of these cations calculated in the absence or in presence of alkali metal bases in the mixing water.